[LTP] [PATCH v3] lib: memutils: respect minimum memory watermark when polluting memory

[Krzysztof Kozlowski]

Previous fix for an out-of-memory killer killing ioctl_sg01 process
in commit 4d2e3d44fad5 ("lib: memutils: don't pollute
entire system memory to avoid OoM") was not fully effective.  While it
covers most of the cases, an ARM64 machine with 128 GB of memory, 64 kB
page size and v5.11 kernel hit it again.  Polluting the memory fails
with OoM:

  LTP: starting ioctl_sg01
  ioctl_sg01 invoked oom-killer: gfp_mask=0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), order=0, oom_score_adj=0
  ...
  Mem-Info:
  active_anon:309 inactive_anon:1964781 isolated_anon:0
                  active_file:94 inactive_file:0 isolated_file:0
                  unevictable:305 dirty:0 writeback:0
                  slab_reclaimable:1510 slab_unreclaimable:5012
                  mapped:115 shmem:339 pagetables:463 bounce:0
                  free:112043 free_pcp:1 free_cma:3159
  Node 0 active_anon:19776kB inactive_anon:125745984kB active_file:6016kB inactive_file:0kB unevictable:19520kB ...
  Node 0 DMA free:710656kB min:205120kB low:256384kB high:307648kB reserved_highatomic:0KB active_anon:0kB inactive_anon:3332032kB ...
  lowmem_reserve[]: 0 0 7908 7908 7908
  Node 0 Normal free:6460096kB min:6463168kB low:8078912kB high:9694656kB reserved_highatomic:0KB active_anon:19776kB inactive_anon:122413952kB ...
  lowmem_reserve[]: 0 0 0 0 0

The important part are details of memory on Node 0 in Normal zone:
1. free memory: 6460096 kB
2. min (minimum watermark): 6463168 kB

Parse the /proc/sys/vm/min_free_kbytes which contains the free
memory level used as minimum watermark (triggering OoM killer).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

---

Changes since v2:
1. Use /proc/sys/vm/min_free_kbytes instead of parsing zoneinfo, thanks
   tgo Liu Xinpeng.

Changes since v1:
1. Add static and rename to count_min_pages().
---
 lib/tst_memutils.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/tst_memutils.c b/lib/tst_memutils.c
index af132bcc6c24..df53c542d239 100644
--- a/lib/tst_memutils.c
+++ b/lib/tst_memutils.c
@@ -16,12 +16,18 @@
 void tst_pollute_memory(size_t maxsize, int fillchar)
 {
 	size_t i, map_count = 0, safety = 0, blocksize = BLOCKSIZE;
+	unsigned long min_free;
 	void **map_blocks;
 	struct sysinfo info;
 
+	SAFE_FILE_SCANF("/proc/sys/vm/min_free_kbytes", "%lu", &min_free);
+	min_free *= 1024;
+	/* Apply a margin because we cannot get below "min" watermark */
+	min_free += min_free / 10;
+
 	SAFE_SYSINFO(&info);
 	safety = MAX(4096 * SAFE_SYSCONF(_SC_PAGESIZE), 128 * 1024 * 1024);
-	safety = MAX(safety, (info.freeram / 64));
+	safety = MAX(safety, min_free);
 	safety /= info.mem_unit;
 
 	if (info.freeswap > safety)
-- 
2.30.2